UNIT - V WAVEGUIDES. Part A (2 marks)

Size: px

Start display at page:

Download "UNIT - V WAVEGUIDES. Part A (2 marks)"

Everett Williamson
5 years ago
Views:

1 Part A (2 marks) UNIT - V WAVEGUIDES 1. What is the need for guide termination? (Nov / Dec 2011) To avoid reflection loss. The termination should provide a wave impedance equal to that of the transmission mode in the guide. 2. A rectangular waveguide of cross section 5 cm X 2 cm is used to propagate TM 11 mode at 10 GHz. Determine the cut-off waveguide. (Nov / Dec 2011) For a = 5 cm, b = 2 cm, m = 1, n = 1 λ c = 2 / ( [m/a] 2 + [n/b] 2 ) = cm. 3. What is the dominant mode of rectangular waveguide? (May / Jun 2012) The wave which has the lowest cutoff frequency is called the dominant wave. In rectangular waveguide, the lowest order of TE wave is TE 10 mode has the lowest cutoff frequency and so TE 10 wave is the dominant wave. 4. Write the expression for the wave impedance and guide wavelength for TEM mode. (May/ Jun 2012) Guide wavelength, λ g = 2π/β = 2π / ω μ 0 ε 0 = (2π. V 0 ) / 2πf = λ = wavelength of free space. Wave impedance is given by, E x / H y = β/ωμ = (ω μ 0 ε 0 )/ ω ε 0 = μ 0 / ε 0 = η = instrinsic impedance of the dielectric medium. 5. Why is the Bessel s function of the second kind not applicable for the field analysis inside the circular waveguide? (Nov / Dec 2008) Bessel s function of second kind is having a property that their values become infinity for all the orders, when its argument is zero. Practically, it is not possible to have an infinite field. Therefore, it is not applicable for the field analysis inside the circular waveguide. 1. A rectangular air filled copper waveguide with dimensions of a = 2.28 cm and b = 1.01 cm has a 9.2 GHz signal propagated in it. Determine the guide wavelength for TE 10 mode. (Nov / Dec 2009) Given : TE 10 mode a = 2.28 cm b = 1.01 cm Cutoff wavelength : λ c = 2 / ( [m/a] 2 + [n/b] 2 ) = 2 / ( [1/ 2.28 x 10-2] 2 + [0/ 1.01] 2 ) = List out the parameters describing the performance of a resonator. (Apr/May 2010)

The performance of resonator are : Resonant frequency Quality factor Input impedance. 3. Calculate the cutoff wavelength for the TM 11 mode in a standard rectangular waveguide if a = 4.5 cm.

2 The performance of resonator are : Resonant frequency Quality factor Input impedance. 3. Calculate the cutoff wavelength for the TM 11 mode in a standard rectangular waveguide if a = 4.5 cm. (Nov/ Dec 2010) (Apr / May 2011) Cutoff wavelength, λ c = 2 / ( [m/a] 2 + [n/b] 2 ) λ c = 2 / ( [1/a] 2 + [1/(a/b)] 2 ) = 4.02 cm. 4. Compare transmission line and waveguide. (Apr / May 2011) Transmission line: May operate from dc to very high frequency. Inefficient due to skin effect and dielectric loss. Waveguide: It can operate only above a certain frequency called cutoff frequency. Larger bandwidth and lower attenuation. Part B (16 marks) 1. Explain in detail about waveguides. Waveguides are basically a device ("a guide") for transporting electromagnetic energy from one region to another. Typically, waveguides are hollow metal tubes (often rectangular or circular in cross section). They are capable of directing power precisely to where it is needed, can handle large amounts of power and function as a high-pass filter. The waveguide acts as a high pass filter in that most of the energy above a certain frequency (the cutoff frequency) will pass through the waveguide, whereas most of the energy that is below the cutoff frequency will be attenuated by the waveguide. Waveguides are often used at microwave frequencies (greater than 300 MHz, with 8 GHz and above being more common). Waveguides are wideband devices, and can carry (or transmit) either power or communication signals. Lets examine metal cavities with a rectangular cross section. Assume the waveguide is filled with vacum, air or some dielectric with the permeability given by and the permittivity given by. The waveguide has a width a in the x-direction, and a height b in the y-direction, with a>b. The z-axis is the direction in which the waveguide is to carry power. Figure 1. Cross section of a waveguide with long dimension a and short dimension b.

3 On this page, going to give the general "rules" for waveguides. That is, it will give the equations for key parameters and let you know what the parameters mean. On the next page, we'll go into the mathematical derivation (which you would do in engineering graduate school), but you can get away with not knowing all that math if you don't want to know it. First and possibly most importantly, this waveguide has a cutoff frequency, fc. The cutoff frequency is the frequency at which all lower frequencies are attenuated by the waveguide, and above the cutoff frequency all higher frequencies propagate within the waveguide. The cutoff frequency defines the high-pass filter characteristic of the waveguide: above this frequency, the waveguide passes power, below this frequency the waveguide attenuates or blocks power. The cutoff frequency depends on the shape and size of the cross section of the waveguide. The larger the waveguide is, the lower the cutoff frequency for that waveguide is. The formula for the cutoff frequency of a rectangular cross sectioned waveguide is given by: In the above, c is the speed of light within the waveguide, mu is the permeability of the material that fills the waveguide, and epsilon is the permittivity of the material that fills the waveguide. Note that the cutoff frequency is independent of the short length b of the waveguide. The cutoff frequency for a waveguide with a circular cross section of radius a is given by: 2. Derive the applications that are restricted to the maxwell s equation. Due to Maxwell's Equations, the fields within the waveguide always have a specific "form" or "waveshape" to them - these are called modes. Assume the waveguide is oriented such that the energy is to be transmitted along the waveguide axis, the z-axis. The modes are classified as either TE ('transverse electric' - which indicates that the E-field is orthogonal to the axis of the waveguide, so that Ez=0) or TM ('transverse magnetic' - which indicates that the H-field is orthogonal to the axis of the waveguide, so Hz = 0). The modes are further classified as TEij, where the i and j indicate the number of wave oscillations for a particular field direction in the long direction (dimension a in Figure 1) and short direction (dimension b in Figure 1), respectively. Metal waveguides cannot support the TEM ('transverse electric and magnetic' - when Ez and Hz are zero) mode. Their exists no solution to Maxwell's equations that also satisfy the required boundary conditions for this mode to occur. Maxwell's Equations are not easy to solve. Hence, every math trick someone can think of will be used in order to make the analysis tractable. We'll start with discussing the electric vector potential, F. In a source-free region (i.e., an area through which waves propagate that is away from sources), we know that: In the above, D is the Electric Flux Density. If a vector quantity is divergenceless (as in the above), then it can be expressed as the curl of another quantity. This means that we can write the solution for D and the corresponding electric field E as:

4 In the above, epsilon is the permittivity of the medium through which the wave propagates. We are purely in the world of mathematics now. The quantity F is not physical, and is of little practical value. It is simply an aid in performing our mathematical manipulations. It turns out that waves (or electromagnetic energy) can not propagate in a waveguide when both Hz and Ez are equal to zero. Hence, what field configurations that are allowed will be classified as either TM (Transverse Magnetic, in which Hz=0) and TE (Transverse Electric, in which Ez=0). The reason that waves cannot be TEM (Transverse Electromagnetic, Hz=Ez=0) will be shown towards the end of this derivation. To perform our analysis, we'll assume that Ez=0 (i.e., we are looking at a TE mode or field configuration). In this case, working through Maxwell's equations, it can be shown that the E- and H- fields can be determined from the following equations: Therefore, if we can find Fz (the z-component of the vector F), then we can find the E- and H- fields. In the above equation, k is the wavenumber. Working through the math of Maxwell's Equations, it can be shown that in a source-free region, the vector potential F must satisfy the vector wave equation: [1] To break this equation down, we will look only at the z-component of the above equation (that is, Fz). We will also assume that we are looking at a single frequency, so that the time dependence is assumed to be of the form given by (we are now using phasors to analyze the equation): Then the equation [1] can be simplified as follows:

To solve this equation, we will use the technique of separation of variables. Here we assume that the function Fz(x, y, z) can be written as the product of three functions, each of a single variable.

variable in the equation (for instance, Z' represents the derivative of the Z-function with respect to z).

5 To solve this equation, we will use the technique of separation of variables. Here we assume that the function Fz(x, y, z) can be written as the product of three functions, each of a single variable. That is, we assume that: Now we plug in our assumption for Fz (equation [3]) into equation [2], and we end up with: [4] In the above equation, the prime represents the derivative with respect to the variable in the equation (for instance, Z' represents the derivative of the Z-function with respect to z). We will break up the variable k^2 into components (again, just to make our math easier): Using equation [5] to breakdown equation [4], we can write: [5] [6] The reason that the equations in [6] are valid is because they are only functions of independent variables - hence, each equation must hold for [5] to be true everywhere in the waveguide. Solving the above equations using ordinary differential equations theory, we get: The form of the solution in the above equation is different for Z(z). The reason is that both forms (that for X and Y, and that for Z), are both equally valid solutions for the differential equations in equation [6]. However, the complex exponential typically represents travelling waves, and the [real] sinusoids represent standing waves. Hence, we choose the forms given in [7] for the solutions. No math rules are violated here; again, we are just choosing forms that will make our analysis easier.

6 For now, we can set c5=0, because we want to analyze waves propagating in the +zdirection. The analysis is identical for waves propagating in the -z-direction, so this is fairly arbitrary. The solution for Fz can be written as: We can write the allowable field configurations for the TE (transverse electric) modes within a waveguide:

7 3. Cut- off frequency for the waveguides: Cutoff Frequency (fc) At this point in the analysis, we are able to say something intelligent. Recall that the components of the wavenumber must satisfy the relationship: Since kx and ky are restrained to only take on certain values, we can plug this fact in: the lowest frequency in which the waveguide will propagate the TE mode. For propagation to occur,. If this is true, then kz is a real number, so that the field components (equations [1] and [2]) will contain complex exponentials, which represent propagating waves. If on the other hand,, then kz will be an imaginary number, in which case the complex exponential above in equations [1-2] becomes a decaying real exponential. In this case, the fields will not propagate but instead quickly die out within the waveguide. Electromagnetic fields that die off instead of propagate are referred to as evanescent waves. To find the lowest frequency in which propagation can occur, we set kz=0. This is the transition between the cutoff region (evanescent) and the propagation region. Setting kz=0 in equation [4], we obtain: If m and n are both zero, then all of the field components in [1-2] become zero, so we cannot have this condition. The lowest value the left hand side of equation [5] can take occurs when m=1 and n=0. The solution to equation [5] when m=1 and n=0, gives the cutoff frequency for this waveguide: Any frequency below the cutoff frequency (fc) will only result in evanescent or decaying modes. The waveguide will not transport energy at these frequencies. In addition, if the waveguide is operating at a frequency just above fc, then the only mode that is a propagating mode will be the TE10 mode. All other modes will be decaying. Hence, the TE10 mode, since it has the lowest cutoff frequency, is referred to as the dominant mode. Every mode that can exist within the waveguide has its own cutoff frequency. That is, for

8 a given mode to propagate, the operating frequency must be above the cutoff frequency for that mode. By solving [5] in a more general form, the cutoff frequency for the TEmn mode is given by: 4. Transverse magnetic waves (TM waves): Determining the fields for the TMz (Transverse Magnetic to the z direction) modes follows a similar procedure to that for the TEz case. To begin, we'll start by discussing the magnetic vector potential, A. This is a non-physical quantity that is often in used antenna theory to simplify the mathematics of Maxwell's Equations. To understand the magnetic vector potential, note that since the magnetic flux density B must always be divergenceless: If a vector quantity is divergenceless, then it can be expressed as the curl of another vector quantity. In math notation, this means that B can be written as: In a source free region, it can be shown that A must satisfy the wave equation: In addition, the TMz fields can be found from the Az component of the magnetic vector potential, via the following relationships:

9 To solve for Az (and hence determine the E- and H- fields), we follow the same procedure as for the TEz case. That is, we use separation of variables and solve the wave equation for the z-component of A, then apply boundary conditions that force the tangential components of the E-fields to be zero on the metallic surfaces. Performing this procedure, which will not be repeated here, we obtain the solution for Az: The corresponding TMz fields for waves propagating in the +z-direction are: 5. Cut off frequency of TM mode and the field distributions of TE and TM waves. In the above equations, k is again the wavenumber, and Bmn is a constant, which determines the amplitude of the mn mode (a function of how much power is applied to the waveguide at that frequency). Before the modes, we note that TM0n and TMm0 modes cannot exist; that is, m and n must be at least 1. The reason comes from equation [1] above - if either m or n are zero, then Az is equal to zero, so all the fields derived must also be zero. Hence, the lowest order mode for the TM case is the TM11 mode. The same procedures can be applied from the TE case to determine the cutoff frequencies for the TMmn mode:

10 MODES:

11 6. Cylindrical waveguides: Cylindrical waveguide Using the complete formulation in the simplest limit possible, global electromagnetic modes are here studied in a large aspect ratio, circular cross-section vacuum cavity equivalent to a cylindrical waveguide. Classical electrodynamics show that the EM eigenmode spectrum consists of two types of solutions, the transverse electric and the transverse magnetic polarizations with frequencies depending on l the radial and m the azimuthal mode numbers. These results are reproduced numerically to verify that the wave equations (16) can indeed be solved in the vacuum using standard LFEM and CFEM discretizations, without introducing spurious modes of numerical origin. It is also important to validate the numerical implementation using a simple test case, checking that the numerical solutions converge to the analytical values with rates expected from the order of the approximations. As the equilibrium merely produces the geometry and the mesh, the safety factor does not affect the eigen frequency spectrum; using a large value for, it is however possible to everywhere align with the axis of the cylinder and separate the components of the TE and the TM polarizations. To compute the eigenmode spectrum, an oscillating source current (eq.22) is driven with a small imaginary part in the excitation frequency The power relation (eq.48) yields a complex response function which has poles along the real axis that correspond to the solutions of the discretized wave equations. The eigen frequencies are calculated by scanning in the complex plane with an increment and a constant chosen so as to resolve the response peaks in. The structure of an eigenmode is obtained in the limit when the cavity is resonantly excited at the maximum of a narrow response peak. In order to verify that the eigenfrequency spectrum of this cylindrical waveguide is complete and does not contain any spurious ''polluting`` mode, two broad scans are performed from 10 khz to 10 GHz with a high resolution in frequency and a low resolution in space for LFEM, for CFEM). All the Fourier modes representable by the numerical discretization are excited with azimuthal currents for TE modes, and axial currents for TM modes. Fig.6 summarizes the result obtained with LFEM, showing that every mode found numerically could be identified in a one to one correspondence with the analytical result. Modes which have low quantum numbers (l,m) are, as expected, obtained with a better precision; pushing the resolution to the lowest limit of 2 mesh points per wavelength (m=4), the deviations become of course important, but the spectrum remains unpolluted (remember fig.3, root b). The same analysis has been repeated with CFEM and leads to results which are much more precise. As an illustration, the eigenmode has here been calculated on a coarse homogeneous mesh. The eigenfrequency obtained numerically GHz is in excellent agreement with the analytical result = GHz; fig.5 shows the eigenmode structure in a vector plot of.

Analytical (circles) and LFEM (x-marks) eigen frequency spectrum. 7. Rectangular cavity resonator: Cavity resonators are enclosed metal boxes. Electromagnetic fields are confined inside the boxes.

12 Analytical (circles) and LFEM (x-marks) eigen frequency spectrum. 7. Rectangular cavity resonator: Cavity resonators are enclosed metal boxes. Electromagnetic fields are confined inside the boxes. Radiation and high-resistance effects are eliminated, resulting in a very high Q (quality factor) A rectangular waveguide with both ends (z=0 and z=d) closed by a conducting wall (Figure 9-8) : multiple reflections and standing waves

13 Resonant frequency of rectangular cavity resonator Degenerate modes : different modes having the same resonant frequency. Dominant mode : the mode with the lowest resonant frequency for a given cavity size. A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical (including acoustic). Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal. Musical instruments use acoustic resonators that produce sound waves of specific tones. A cavity resonator, usually used in reference to electromagnetic resonators, is one in which waves exist in a hollow space inside the device. Acoustic cavity resonators, in which sound is produced by air vibrating in a cavity with one opening, are known as Helmholtz resonators. 8. Principles Cavity resonators: Cavity resonators A cavity resonator is a hollow conductor blocked at both ends and along which an electromagnetic wave can be supported. It can be viewed as a waveguide short-circuited at both ends (see Microwave cavity). The cavity has interior surfaces which reflect a wave of a specific frequency. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low loss (see standing wave). As more wave energy enters the cavity, it combines with and reinforces the standing wave, increasing its intensity. The cavity magnetron is a vacuum tube with a filament in the center of an evacuated, lobed, circular cavity resonator. A perpendicular magnetic field is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode. Spaced about the rim of the chamber are cylindrical cavities. The cavities are open along their length and so connect the common cavity space. As electrons sweep past these openings they induce a resonant high frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. A portion of this field is extracted with a short antenna that is connected to a waveguide (a metal tube usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high gain antenna in the case of radar. The klystron, tube waveguide, is a beam tube including at least two apertured cavity resonators. The beam of charged particles passes through the apertures of the resonators, often tunable wave reflection grids, in succession. A collector electrode is provided to intercept the beam after passing through the resonators. The first resonator causes bunching of the particles passing through it. The bunched particles travel in a field-free region where further bunching occurs, then the bunched particles enter the second resonator giving up their energy to excite it into oscillations. It is a particle accelerator that works in conjunction with a specifically tuned cavity by the configuration of the structures. On the beam line of an accelerator system, there are specific sections that are cavity resonators for RF. The reflex klystron is a klystron utilizing only a single apertured cavity resonator through which the beam of charged particles passes, first in one direction. A repeller electrode is provided to repel (or redirect) the beam after passage through the resonator back through the resonator in the other direction and in proper phase to reinforce the oscillations set up in the resonator.

14 In a laser, light is amplified in a cavity resonator which is usually composed of two or more mirrors. Thus an optical cavity, also known as a resonator, is a cavity with walls which reflect electromagnetic waves (light). This will allow standing wave modes to exist with little loss outside the cavity.

18 MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI EC 2305 TRANSMISSION LINES AND WAVEGUIDES -V SEM ECE

Waveguides. Metal Waveguides. Dielectric Waveguides

Waveguides. Metal Waveguides. Dielectric Waveguides Waveguides Waveguides, like transmission lines, are structures used to guide electromagnetic waves from point to point. However, the fundamental characteristics of waveguide and transmission line waves